In the operation of Joule-Thompson minicoolers it has been found desirable to use two different gases in sequence, the first gas being used to enable a low temperature to be obtained quickly, and the second gas being used to maintain that temperature during the operating period. One way of providing two gases in sequence is to use a separate gas bottle for each, in combination with a suitably controlled system of valves. However, this is expensive and complicated, and the resulting system also tends to be heavy--something which is very undesirable in many situations.
It is an aim of the present invention at least to alleviate these problems. It is a further aim of the present invention to provide a light-weight, relatively simple pressure vessel which will automatically discharge a controlled amount of a first gas followed by a controlled amount of a second gas.